Improve message if file exists instead of folder

When TemporaryFolder(String path) or TemporaryFolder(String... paths) is
called with a path that matches the path of an existing file then an
IOException with a message like "a file with path <path> exists" is
thrown.

Author: stefanbirkner

src/main/java/org/junit/rules/TemporaryFolder.java

@@ -206,8 +206,13 @@ public File newFolder(String... paths) throws IOException {
 
             lastMkdirsCallSuccessful = file.mkdirs();
             if (!lastMkdirsCallSuccessful && !file.isDirectory()) {
-                throw new IOException(
-                        "could not create a folder with the path \'" + relativePath.getPath() + "\'");
+                if (file.exists()) {
+                    throw new IOException(
+                            "a file with the path \'" + relativePath.getPath() + "\' exists");
+                } else {
+                    throw new IOException(
+                            "could not create a folder with the path \'" + relativePath.getPath() + "\'");
+                }
             }
         }
         if (!lastMkdirsCallSuccessful) {

src/test/java/org/junit/rules/TemporaryFolderUsageTest.java

@@ -90,7 +90,7 @@ public void newFolderWithGivenFolderThrowsIOExceptionIfFileExists() throws IOExc
         assertTrue("Could not create" + file, file.createNewFile());
 
         thrown.expect(IOException.class);
-        thrown.expectMessage("could not create a folder with the path 'level1'");
+        thrown.expectMessage("a file with the path 'level1' exists");
         tempFolder.newFolder("level1");
     }